The present invention relates generally to acoustic levitation and concentration and, more particularly, to the use of a hollow, oblate piezoelectric crystal for levitation and concentration which requires only low power and obviates the need for exact alignment of parts to generate standing waves.
Acoustic levitation provides a means for isolating small samples of particles having diameters less than several millimeters without the influence of a containment vessel (See, e.g., E. H. Trinh, xe2x80x9cCompact Acoustic Levitation Device For Studies In Fluid Dynamics And Material Science In The Laboratory And Microgravityxe2x80x9d Rev. Sci. Instrum. 56, 2059-2065 (1985), D. B. Thiessen and P. L. Marston, xe2x80x9cPrinciples Of Some Acoustical, Electrical, And Optical Manipulation Methods With Applications To Drops, Bubbles, And Capillary Bridgesxe2x80x9d ASME Fluids Eng. Div. Publ. FED (1998), M. A. H. Weiser and R. E. Apfel, xe2x80x9cExtension Of Acoustic Levitation To Include The Study Of Micron-Size Particles In A More Compressible Host Liquidxe2x80x9d J. Acoust. Soc. Am. 71, 1261-1268 (1982), E. G. Lierke et al., xe2x80x9cAcoustic Positioning For Space Processing Of Materials Science Samples In Mirror Furnacesxe2x80x9d in IEEE Ultrasonics Symposium 1129-1139 (1983), K. Yasuda, xe2x80x9cBlood Concentration By Superposition Of Higher Harmonics Of Ultrasoundxe2x80x9d Jpn. J. Appl. Phys. 36, 3130-3135 (1997), Ph. Caperan et al., xe2x80x9cAcoustic Agglomeration Of A Glycol For Aerosol: Influence Of Particle Concentration And Intensity Of The Sound Field At Two Frequenciesxe2x80x9d J. Aerosol Sci. 26, 595-612 (1995), G. Whitworth et al., xe2x80x9cTransport And Harvesting Of Suspended Particles Using Modulated Ultrasoundxe2x80x9d Ultrasonics 29, 439-444 (1991), and K. M. Martin and O. A. Ezekoye, xe2x80x9cAcoustic Filtration And Sedimentation Of Soot Particlesxe2x80x9d Experiments in Fluids 23, 483-488 (1997)). Most acoustic levitation devices operate by localizing a sample near the nodal planes of an acoustic standing wave. This has proven to be a viable technique for measuring material properties of small sample quantities (e.g. droplets, aerosols, etc.) without obscuring the results with the effects of a mounting apparatus (See, e.g., M. A. H. Weiser and R. E. Apfel, supra, and E. G. Lierke et al., supra). Other applications include the use of acoustic forces to concentrate aerosols and/or particulates near the nodal planes of the field for harvesting or sedimentation purposes. Advances in the design of acoustic levitators over the past several decades have proven useful for applications where samples may reside in either gaseous or liquid host media.
The standing-wave field produced by an acoustic levitation device is strongly dependent upon the spatial alignment of the system components and often requires moderate to high electrical input power levels to drive the acoustic generators and achieve the desired levitation. This is especially true for levitating solid and liquid samples in air. The large acoustic impedance mismatch between the displacement-generating device and the air medium is often a difficult problem to overcome. Resonant transduction devices having Q greater than 1000 have been built to address this problem and have proven quite useful when electrical power efficiency is not a limiting factor (See, e.g., D. B. Thiessen and P. L. Marston, supra, and J. A. Gallego Juarez and G. Rodriguez Corral xe2x80x9cPiezoelectric Transducer For Air-Borne Ultrasoundxe2x80x9d Acustica 29, 234-239 (1973)).
Piezoelectric cylinders have received significant attention by industry. Such crystals are used for vibration damping, sources for sonar, sensors and actuators, motors and X-Y micropositioners, to name several uses.
Accordingly, it is an object of the present invention to provide an apparatus for efficiently achieving acoustic levitation and concentration which in its simplest embodiment is free from the requirement of careful alignment of its component members.
Various advantages and novel features of the invention will be set forth, in part, in the description that follows, and, in part, will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.
In accordance with the present invention, particles suspended or entrained in a fluid are concentrated. A cylindrical piezoelectric transducer is provided with an interior cylindrical cavity having a surface defining an axis and having a resonance frequency matched to the breathing mode frequency of the transducer. The interior cylindrical cavity surface is deformed from a circular cross-section such that applying a periodic electrical excitation to the cylindrical piezoelectric transducer generates resonant acoustic waves within the interior cylindrical cavity to form localized force concentration regions of acoustic force parallel to the axis of the cavity surface. The fluid having particles suspended or entrained therein is subjected to the localized force concentration regions such that the particles move to the localized force concentration regions and are concentrated therein.
In another aspect of the present invention, an apparatus concentrates particles suspended or entrained in a fluid. A cylindrical piezoelectric transducer has an interior cavity having a surface defining an axis, wherein the interior cavity has an acoustic resonance that is matched to a breathing-mode acoustic resonance of the cylindrical piezoelectric transducer when the interior portion is filled with the fluid. A function generator is connected to apply periodic electrical excitation to the cylindrical piezoelectric transducer such that resonant acoustic waves are generated in the interior cavity and form regions of concentrated acoustic force, where the interior cavity surface is deformed from a circular cross-section so that the regions of concentrated acoustic force are axial regions parallel to the axis of the interior cavity. An input system introduces the fluid having suspended or entrained particles adjacent the regions of concentrated acoustic force formed by the resonant acoustic waves such that the particles move to the regions and are concentrated therein.